MEMS devices are used, for example, as airbag accelerometers, microengines, optical switches, gyroscopic devices, sensors, and actuators. MEMS devices are typically manufactured from a silicon wafer using microlithography, and have freestanding active silicon elements (e.g., gears, hinges, levers, slides, and mirrors) that must be free to move, rotate, etc. The inherent mechanical nature of MEMS devices brings about a high and ever increasing level of complexity to their production and reliability.
MEMS devices generally have large surface-area-to-volume ratios, and thus their performance is often dominated by strong adhesion (i.e., stiction) of active silicon elements to one another and/or to the supporting silicon substrate. Stiction may be caused by capillary forces (e.g., resulting from high critical surface tension of silicon surfaces of MEMS devices), electrostatic forces, van der Waals forces, and/or “chemical” forces such as hydrogen bonding and solid bridging. Stiction may occur, for example, during release of MEMS elements from via etching, or at a later time (e.g., during use).
Stiction, friction, and wear are major problems limiting both the production yield and useful lifetime of many MEMS devices, and have plagued the MEMS industry since its inception.
To address the problem of stiction, various treatments that reduce stiction (i.e., anti-stiction treatments) are commonly employed. Such treatments typically reduce capillary forces by reducing the surface energy of MEMS surfaces (e.g., resulting in a high advancing and/or receding contact angle with water). Ideally, anti-stiction treatments are stable at typical operating temperatures of MEMS devices (e.g., in the range of from −40 to +130° C.). In some cases, an anti-stiction treatment may also serve to lubricate moving MEMS elements, thereby ensuring proper function and reducing the rate of wear.
Many anti-stiction and/or lubrication treatments are mere liquid coatings, and thus prone to loss or removal over time. Other anti-stiction treatments utilize various grafting techniques to chemically bond an anti-stiction coating to silicon surfaces of the MEMS device. However, and notwithstanding any reduction in stiction that may be achieved by such methods, there continues to be a need for further improvements in anti-stiction treatments.